This is to clear up the enormous confusion I've noticed in the last few months about dual 12V line current specs on PSUs.

SPECS

Version 2.0 of Intel's ATX12V Power Supply Design Guide began recommending dual 12V lines for PSUs that can deliver more than 18A at 12V. Why? To abide by safety requirements of UL and EM 60950, which stipulates not more than 240VA on any wires or exposed traces. Intel's PSU Guide calls for a current limiter that keeps current to under 20A on each of the 12V rails: 12V x 20A = 240VA.

What is the safety reason for this 240VA maximum? It's the maximum recommended for an electronic device that a consumer will have reasonable likelihood of access. In plain terms, it might be to keep people from zapping themselves inside a PC, or more likely, accidentally creating a fire risk. This safety "rule" does not apply to any electronic or electrical devices where the chance of consumer exposure is low, such as a TV or CRT monitor, for example.

It's important to remember that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be much too costly and inefficient. There is only one 12VDC source, and the two lines draw from the same transformer. Each line is coming from the same 12VDC source, but through its own "controlled gateway".

PSU makers' specs are misleading in that thay rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.

What the above means is that you don't need to worry about imbalances in power draw on the 12V lines —as long as no single rail is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis, probably more for marketing reasons than any other. A PSU rated for 32A max on the 12V lines can be labelled many different ways:

It could be marked 20A + 12A, but being a cautious bunch, the engineers will probably not specify more than 18A on any one line. This gives 2A headroom to allow some room for error for the current limiting circuit.

REALITIES

Note that 12V2 is supposed to supply only the AUX12V (2x12V) 4-pin plug, which feeds only the CPU. With PSUs that adhere strictly to the ATX 12V v2.xx Guide, 12V1 supplies 12V to all the other components that require it. This might lead to a problem with very high power gaming systems that utilize two high power video cards. Current high end VGA cards by themselves can draw >90VA each. Much of this comes from the 12V line via the 6-pin PCIe connector for the VGA card. If you add several hard drives and optical drives, the 240VA limit may be too low.

The current ATX12V v2.2 spec was created before dual VGA card gaming configurations for Intel boards were announced. SLI, being an AMD feature that came many months earlier, may have been ignored by Intel's PSU design guide team.

I interviewed a number of engineers from several power supply manufacturers to pose this very question. The answers were surprising. All of the engineers I spoke with wished to remain anonymous. This is a summary of what they told me:

**Some PSU makers are using 12V2 to supply more than just the 2x12V or 4x12V connectors. It is often used to power the 6-pin 12V PCIe outputs.

**Many PSUs marked as having dual 12V lines actually have only a single 12V line — they do not feature two <240VA power limiters specified by ATX12V v2.xx; they have only one Over Current Protection (current limiter) for the single 12V line.

**The 240VA current limit is considered a high cost, useless annoyance by most PSU makers. If multiple 12V lines are used, because the vast majority of components now use mostly 12V, the 18~20A limit for any line means that the precise power distribution to the various 12V output connectors can become critically important in some cases.

**The engineers point to the many high power pre-V2.xx ATX12V PSUs that had as much as 30A on a single 12V line. As a product class, those have not proven to be any more dangerous in any way than other ATX12V PSUs.

What's really interesting is that Intel has tacitly waived the 240VA limit requirement in its PSU validation program for the better part of a year.

For the 32 ATX12V v2.2 PSUs tested in 2005 that are on this list, 17 models are identified as having at least one output line that exceeds 240VA. And yet, these 17 models are on Intel's approved list. According to the engineers I spoke with, the majority of these 17 models have just one 12V line. They also point out that there are another 20 or so ATX12V v2.0 PSU models on the Intel list, and none of them were tested for the 240VA current limit conformance. My sources say that if these models had been tested, more than half would not conform to the 240VA current limit because they have only one 12V line.

Now, my sources say, in the last couple of months, Intel has notified the PSU makers verbally that the 240VA current limit has officially been removed. This means a single 12V line is the accepted norm, never mind ATX12V v2.xx.

What does all this mean? Essentially, the only potential benefit of dual 12V lines is improved safety, and this is disputed by the engineers I spoke with. There are many downsides to dual 12V lines, including higher cost and the extra worry of ensuring adequate 12V current for all the components in complex, high power systems. For the consumer who is trying to make a choice among the myriad of PSUs available on the retail market today, the most practical approach regarding dual 12V lines and power capacity is to consider only the combined 12V current capacity.

It only takes 0.1A to kill a human being.... how is limiting it to 20A "safe"?

First response... and it's off topic.

This really isn't the issue here, you'd have to study the UL safety codes to know. Regardless of how much current is enough to kill a person, I've never heard a single story of anyone being killed or even hurt by getting zapped while working inside a PC. I doubt protection from electric shock is the main issue. Perhaps more to do with heat? I really can't say.

In any case, 240VA is the specified maximum. Here are the pertinent references in Intel ATX12V v2.2 guide:

-- In cases where expected current requirements is greater than 18A a second 12 V rail should be made available. (18x12=216VA, to allow peak headroom to 240VA)-- The 12V rail on the 2 x 2 power connector should be a separate current limited output to meet the requirements of UL and EN 60950.
-- 12V1DC and 12V2DC should have separate current limit circuits to meet 240VA safety requirements.
-- 12V2DC supports processor power requirements and must have a separate current limit and provide 16.5A peak current for 10 ms; minimum voltage during peak is > 11.0 VDC

Mike, your initial post is a little confusing to me. You're suggesting to me that with a PSU like the Seasonic S12-330 (12v1: 8A, 12v2: 14A), that we can interpret this to be essentially 22A @ 12V. This is seems like a dangerous assumption to make.

The motherboard is the only place that is supposed to be connected to 12v2 (assuming it's ATX 2.xx compliant). That means that all your other peripherals that are not directly connected to the motherboard receives its power from 12v1. This includes any external power connector for video cards.

So a well thought question on multiple rails then becomes, how much power does a video card draw and from which lines? Normally, we assume the worst case scenario (all of it on 12v1) and use that as our basis for choosing a PSU.

But from what I'm inferring from your post, that may not be a valid assumption.

Mike, your initial post is a little confusing to me. You're suggesting to me that with a PSU like the Seasonic S12-330 (12v1: 8A, 12v2: 14A), that we can interpret this to be essentially 22A @ 12V. This is seems like a dangerous assumption to make.

This is precisely correct, but only <20A is available from either 12V rail, and the exact proportion simply depends on how much is demanded from each rail. So if the CPU is a 70W TDP Northwood like the P4-2.8 in my main rig, that accounts for just 6A, which means that up to 16A is available for the rest oif the system.

Quote:

The motherboard is the only place that is supposed to be connected to 12v2 (assuming it's ATX 2.xx compliant). That means that all your other peripherals that are not directly connected to the motherboard receives its power from 12v1. This includes any external power connector for video cards.

The 12x2 is really just an extra cable run from the 12V tap, with another limiter on this rail to keep the current in this cable below 240VA. Note: All the ground leads on the PSU are common and it's perfectly safe to join the outputs of 12V1 and 12V2 in parallel -- that's what SPCR's power loader does and we've never had a single failure due to this. It confirms what I've been saying -- the 12V rails all come from the same place, it's only the current limiter that limits how much for each line. With most PSUs, the same <240VA current limiting is applied to all the 12V rails. Anything else just does not make sense.

Quote:

So a well thought question on multiple rails then becomes, how much power does a video card draw and from which lines? Normally, we assume the worst case scenario (all of it on 12v1) and use that as our basis for choosing a PSU.

How much power depends on the card. Where from is 12V2, which also includes the main ATX cable all the 4-pin peripheral power connectors -- the current in all this is limited to 240VA as well. See response to first comment above.

Thank you so much Mike. I've been looking for a clear and consice explanation of this. Your explanation makes a lot of sense.

Ned

You're welcome Ned. It's funny because just before I received notification of your post here, I was rewriting my original post for clairty and to add a bit more info after I had a chat with an Intel power engineer today. These changes are there now.

So if the CPU is a 70W TDP Northwood like the P4-2.8 in my main rig, that accounts for just 6A

just a nitpick, but isn't that only assumming 100% efficiency of the power regulators on the board? i'm guessing they're fairly efficient, on the order of 80-90%, but a little bit of power does get dissipated as heat.

It only takes 0.1A to kill a human being.... how is limiting it to 20A "safe"?

just to clarify, there is no way you will get 20A through a human body at 12V. the resistance is too high. in fact, you will never get electrocuted by 12V, period. you'd barely be able to feel it.

to get 0.1A through a potentially fatal path through the human body (i.e. one that travels near the brain or heart), you'd need significant voltage, particularly at DC. (AC passes more easily through human tissue and is more disruptive of the nervous system.) if you assume a 10kOhm DCR through the chest area (which is on the very low side, unless your skin is wet), you'd need at least 1000V.

limiting the current to 20A on a 12V line for safety reasons is to reduce the fire hazard in case of a short. it has nothing to do with electrocution.

OK, dorkus, I stand corrected. Not getting zapped is what my Intel power eng copnjectured.

oh, not really trying to correct you Mike - the 240VAC limit UL imposes is just a general guideline (and possibly a very old one at that). not getting zapped is indeed probably one of the motivations behind that rule... i was just commenting that in this particular application, it's more likely to reduce the fire hazard.

back on topic... i rewired my older Enermax 350W power supply a couple years ago, and i can't remember if all the 12V lines were tied to the same bus inside... i think they were, which of course supports your point. was the Intel engineer saying that *all* power supplies are like this, with the 12V rails tied together at the source? i would have thought some supplies actually had separate regulators for the different lines (sometimes it's actually easier to use two smaller regulators than one big one), but maybe in the PSU world this isn't the case.

...was the Intel engineer saying that *all* power supplies are like this, with the 12V rails tied together at the source? i would have thought some supplies actually had separate regulators for the different lines (sometimes it's actually easier to use two smaller regulators than one big one), but maybe in the PSU world this isn't the case.

He didn't say *all* but the vast majority. Certainly for retail ATX PSUs, this is true. In the server world, there tends to be much more customization for specific apps and cleints -- which is not difficult when a client orders thousands of one type.

So dual rail is only a matter of maximum allowed current flowing in a wire.
It has some sense, there shouldn't be more than a fixed amount of current in a wire.
If I am not mistaken, I saw a on/off button in the last Tagan power supply, so you should enable it when the system gets hungrier (like SLI) and let it disabled when the system is not.
18 AWG normal, 22 AWG for floppy and 16 AWG for ATX connector in my Zalman (that's why the connectod has burned on my 9700pro when I overclocked it to 400Mhz core with some V.AGP increase :\ )
As you can see here, http://www.powerstream.com/Wire_Size.htm16A typical for a 18 AWG (a little less than the 18A stated here)

So.. I should say it is only a matter of marketing, or am I wrong?
20A for 12V is about 240W of drained power, neither two SLI 6800ultra can take all that amount of power (they are about 100+W each)

Sept 17 -- Much of what's been written here up to now is defunct. Recent long chats with PS engineers have cleared up a lot of the confusion about multiple 12V lines, the 240VA current limit, etc.

In essence, Intel has removed the 240VA current limit on the PS output lines, which means that singe 12V lines will become the norm. The silly thing is that this has been the norm even for more than half the so-called dual 12V line PSUs for the past year.

Great article. I just want to get a clarification on something. Let's say we have these two PSUs:

PSU1- Single 12V line, 32A
PSU2- Dual lines, 12V1: 18A, 12V2: 14A

Assuming the dual lines draw from the same source like you said, is there any advantage to either one of these over the other? Assuming these two PSUs cost the same and are of identical quality, which one would be the better purchase?

Great article. I just want to get a clarification on something. Let's say we have these two PSUs:

PSU1- Single 12V line, 32APSU2- Dual lines, 12V1: 18A, 12V2: 14A

Assuming the dual lines draw from the same source like you said, is there any advantage to either one of these over the other? Assuming these two PSUs cost the same and are of identical quality, which one would be the better purchase?

Thanks.

My choice would be PSU1 just in case PSU2's 12V lines really do have current limiters at 18A and 14A (highly unlikely, but...). If PSU2 really was like this, then it would be a pain having to watch my calories on one side or the other or both...

In truth, Intel's engineers were far too safety conscious and not thinking this through. This thing about 240VA... is it really possible to have that much current go through any single lead in the PC?

A worst case scenario:

Let's assume you have...
* 6 HDD, each pulling 1A on the 12V (and some on the 3.3V line) = 72VA.
* A hot CPU pulling 12A on the AUX12V plug = 144VA
* SLI dual-VGA card setup, each pulling 6A on the 12V line = 72VA + 72VA
* Add another 4A on the 12V line for this and that = 48VA

The total VA on the 12V line (the most stressed in modern systems), if you could max the load on all the components simultaneously (which you can't) is 408VA, is way over 240VA. BUT... how much juice actually goes through any single lead?

Assuming there's a max of 3 4-pin molex plugs on each leadout, that's 36VA to the drives. No worries.

The CPU gets all its power from the 2x12V AUX lead -- and there's a pair of 12V lines in there... so no more than 72VA on each 12V line. No worries.

The VGA cards mostly get their power from the 6-pin PCIe-VGA plug -- and we have 72VA for each, max. No worries.

The rest -- 48VA -- added to any of the above lines -- still keeps us way below 240VA.

In fact, I can't imagine any one line with 240VA anywhere in the PC that's "exposed" to you and me when you take the cover off. It just does not seem feasible.

So where's the 240VA risk that Intel saw in the first place? Inside the PSU?? Surely that is no more exposed to the consumer than the innards of a TV where there is far more dangerous currents/voltage?

I was just reading through the EPS12V 2.9 specs, and I found a couple of relevant quotations that may shed some light on the issue of multiple rails for +12V.

First:

Quote:

If 240VA limiting is not a requirement for the power supply than all +12V outputs are common and may have the same wire color (yellow).

(footnote on p.15)

A little more detective work led me to the EPS12V opinion on 240VA limiting:

Quote:

7.2 240VA ProtectionSTATUS: RecommendedSystem designs may require user access to energized areas of the system. In these cases the power supply may be required to meet regulatory 240VA energy limits for any power rail. Since the +12V rail combined power exceeds 240VA it must be divided into separate channels to meet this requirement. Each separate rail needs to be limited to less than 20A for each +12V rail. The separate +12V rails do not necessarily need to be independently regulated outputs. They can share a common power conversion stage. The +12V rail is split into four rails. Refer to section 6.4 for how the 12V rail is split between different output connectors.

(p.33)

This confirms what MikeC posted originally: the "separate" +12V are only separate in the sense that they have independent over current protection circuits; they are all fed from the same transformer, and are regulated together, not separately.

Even more interesting, the 240VA current limiting is a recommended feature of the spec, not a required feature. This implies that a power supply can fully comply with the required sections of EPS12V without using multiple voltage rails.

A quick glance at the required over current protection reveals that the OCP need only kick in when the current is at 150% of the rated maximum for +12V. Note that it is 150% of the total +12V capacity, not the capacity for the separate rails. Separate OCP circuits for the various +12V rails is only specified in the (not required) 240VA current limiting section that immediately follows the OCP section.

To summarize the most important parts of this post: Multiple +12V lines are recommended, but not required for EPS12V 2.9 power supplies. Power supplies that have multiple +12V lines do not regulate the lines separately; the lines are only separate in the sense that they have independent protection circuits.

Hi, new here, got a question. Don't think it was quite answered in this thread, but

I have a PS with dual 12v rails. The 12V connector on my motherboard is 4 pins (which is standard on all ATX boards, I think). The main power connector is 24 pins and my mobo is 24 pin, so they match up.

The power supply 12V connector has 8 pins, which im assuming is the dual rails (4 each). However, since my mobo's 12V connector only has 4 pins, I can only use 4 pins from the power supply connector. So my question is, am I limiting myself by using this psu? I've never seen a motherboard that has 8 pinholes for a 12V power connector? Can someone explain this? Thanks.

So if dual +12V rails is dead, why is the market still flooded with them?

For example if you look at the two OCZ PowerStream PSU you'll see the 520W unit only has one +12V rail. But if you look at the 600W version, it has two. From what I read here it seems like two rails would be a bad thing, that is if my system drew over 20A on a single rail. I realize that this probably won't happen but I'm just one of those guys who wants to be able to, you know, just in case.

So if dual +12V rails is dead, why is the market still flooded with them?

This is naive.

Intel has not officially changed the AXT12V PSU guide, and as far the general public knows, v2.2 is current. Is there a single PSU maker that would risk losing sales today by NOT specifying dual 12V lines while v2.2 remains in place? Except for those who have read this thread (and other PSU-related articles at SPCR) carefully, the vast majority of "PC enthusiasts" believe that two 12V lines are better than one. The PSU makers are well aware of this.

It's highly unlikely that any mfg would go back to single 12V lines for existing dual 12V line modesl. The change would be way too costly -- think of not only the engineering and production changes, but also all the labels, boxes, spec sheets, etc.

I don't think you will see any change in the dual 12V models out now -- or even in new models -- until & unless Intel changes the ATX12V spec. (Even with models that only claim they have dual 12V lines.)

The info about Intel telling PSU makers that a single 12V line is OK -- this is an insider's insight. They are not telling anyone else but their "partners" in the industry. Remember, you heard it first here.

Any idea where Seasonic stands on this? The confusion about dual 12v rails was part of my decision process for buying a S12-430 instead of a S12-380 for use with a ASUS SLI (AMD Nforce4) motherboard.

I still haven't populated the 2nd AGP slot but I'm assuming if I choose carefully I can do SLI without going over the limits of a mid range S12 Power Supply.

Not sure what you mean by the question... where do they stand on this? How about on two legs?

Seriously... There's nothing any mfg has to do or not do here. Some are adding OCP on two sets of 12V outputs; others are leaving a single 12V line in place with OCP protection for the line. Both are claiming to have two 12V lines. Whether they do or not is moot. The main thing is simply to ensure the total current capacity of the 12V lines is not exceeded. As something like 75~80% of total power deliverable by most ATX12V v2.xx PSUs is on the 12V lines, almost any >400W PSU will do fine for most dual SLI systems, imo. If you need two 6-pin PCIe 12V power cconnectors, a psu equipped w/ them is far more convenient than using adapters...

Any idea where Seasonic stands on this? The confusion about dual 12v rails was part of my decision process for buying a S12-430 instead of a S12-380 for use with a ASUS SLI (AMD Nforce4) motherboard.

I still haven't populated the 2nd AGP slot but I'm assuming if I choose carefully I can do SLI without going over the limits of a mid range S12 Power Supply.

almost any >400W PSU will do fine for most dual SLI systems, imo. If you need two 6-pin PCIe 12V power cconnectors, a psu equipped w/ them is far more convenient than using adapters...

Yes but the S12 500 and S12 600 cost significantly more and you always preach about rightsizing the Power Supply to hit the right part of the effeciency curve.

If the S12 430, 380, or 330 is closer to the sweet spot on avg power consumed and thus uses less electricity (and thus less cooling is needed) isn't it worth dealing with the power adapters for the video cards to:

Heck if I could run two 6800GTs, a ASUS A8NSLI deluxe, 2 optical drives, 2 hard drives, and 5 low speed fans on a S12-380 or S12-330 it would have saved me even more money up front. I just took a guess and went with the 430 not knowing what video cards I'd end up buying.

Do you not see the conundrum in buying a Seasonic S12 with your cumalitive advice? It's not the end of the world, but if you are going to recommend Seasonic and recommend buying lower wattage power supplies and recommend getting a power supply that has 2 PCIe power connectors...

Life is complicated. I don't expect it to be a simple answer, but that doesn't mean I won't ask the questions.

I hope you can see where I'm coming from and not take offense at me asking for clarification.

Yes but the S12 500 and S12 600 cost significantly more and you always preach about rightsizing the Power Supply to hit the right part of the effeciency curve....

....Do you not see the conundrum in buying a Seasonic S12 with your cumalitive advice? It's not the end of the world, but if you are going to recommend Seasonic and recommend buying lower wattage power supplies and recommend getting a power supply that has 2 PCIe power connectors...

Life is complicated. I don't expect it to be a simple answer, but that doesn't mean I won't ask the questions.

I hope you can see where I'm coming from and not take offense at me asking for clarification.

Fair enough.

One thing to point out right away is that the efficiency curves for the PSUs in the S12 line at typical power loads -- say 90W~250W -- are very close. There would be no appreciable difference, even in the long term, between going with one or any other of these PSUs except if you have a really power hungry system -- the 500 and 600 are slightly more efficient at higer loads. Please examine the data for yourself.

The main issues here are overall noise level and initial buying cost -- both higher w/the 500/600. $$ or energy savings doesn't really enter the picture -- or it's just too small to take seriously.

Don't forget that buying adaptors etc. has an intial energy cost that it carries from the manufacturing process. In cases where the efficiency difference is so small, using adaptors may actually increase the energy requirements over the life of the power supply.

You also have to take into account voltage drops caused by the adaptors, which cause more power to be drawn. I think when you're talking about <2-3% differences, there's not much benefit in choosing one over the other.

One thing to point out right away is that the efficiency curves for the PSUs in the S12 line at typical power loads -- say 90W~250W -- are very close. There would be no appreciable difference, even in the long term, between going with one or any other of these PSUs except if you have a really power hungry system -- the 500 and 600 are slightly more efficient at higer loads. Please examine the data for yourself.

The main issues here are overall noise level and initial buying cost -- both higher w/the 500/600. $$ or energy savings doesn't really enter the picture -- or it's just too small to take seriously.

Trust me I've read and reread your articles every couple of months over the last few years. I am not above forgetting so I tend to go back to an article when I'm thinking about it or reading about something else on another site and want to compare.

So the end result is Initial cost is higher with the S12 500 and noise is higher but convience is higher as well.

I'll know by fan noise, system temps/voltages, and/or system stability if I ever out grow the S12-430.

If I choose to get a 500 it won't be a big difference other than the convienence factor.

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